Method of verifying the authenticity of a document and identifiable document produced thereby

ABSTRACT

The invention disclosed herein pertains to a means of verifying the authenticity of a document, identification card, or any object for which a means of establishing authenticity is desired, whereby such document, I.D. card or other object, hereinafter referred to generically as &#39;&#39;&#39;&#39;document,&#39;&#39;&#39;&#39; is treated with a suitable thermoluminescent phosphor in a coded manner and for subsequent verification is exposed to a means for detecting the presence of said thermoluminescent phosphor and for deciphering the message thereby embodies in such document.

United States Patent Harshaw, II

METHOD OF VERIFYING THE AUTHENTICITY OF A DOCUMENT AND IDENTIFIABLEDOCUMENT PRODUCED THEREBY Inventor: William A. Harshaw, H, 6020 Deer us.Cl 2s0 337, 250/83 CD Int. Cl. G0lt 11/11 Field of Search 250/71 R, 83CD References Cited UNITED STATES PATENTS 3/1972 Wheeler 250/71 R 4/1956Rajchman et al.... 250/65 R X 5/1972 Shaw 250/71 R Mar. 26, 19743,639,762 2/1972 Hughes 250/71 R Primary Examiner-James W. LawrenceAssistant Examiner--Davis L. Willis Attorney, Agent, or Firm-Walter J.Monacelli [57] ABSTRACT The invention disclosed herein pertains to ameans of verifying the authenticity of a document, identification card,or any object for which a means of establishing authenticity is desired,whereby such document, ID. card or other object, hereinafter referred togenerically as document, is treated with a suitable thermoluminescentphosphor in a coded manner and for subsequent verification is exposed toa means for detecting the presence of said thermoluminescent phosphorand for deciphering the message thereby embodies in such document.

10 Claims, No Drawings METHOD OF VERIFYING THE AUTHENTICITY OF ADOCUMENT AND IDENTIFIABLE DOCUMENT PRODUCED THEREBY BACKGROUND OF THEINVENTION 1. Field of the Invention The invention relates to a method ormeans of verifying the authenticity of a document. More specifically, itrelates to a means of identifying and verifying the identification of adocument by embodying a coded message in said document in the form of athermoluminescent phosphor and subsequently detecting and decipheringthe embodied message.

2. Related Prior Art Phosphors of the type which respond to excitationby ultraviolet, or so-called black light, emit visible light orinfra-red light which can be detected in the former case by the nakedeye, or in both cases by suitable radiation-sensitive instruments, havebeen used in the past to identify documents or objects which have beendeliberately marked with a pattern or design.

Such types of phosphors have the disadvantage of being relativelyobvious to a would-be forger or counterfeiter, since the light output isspontaneous and concurrent with the application of the exciting orstimulating radiation, and some have a persistent output, or afterglow,after the stimulating radiation has been removed. They also have thedisadvantage of being subject to accidental contamination by the manysubstances which exhibit spontaneous fluorescence to ultra-violet lightor other shorter wave length sources. Such contaminating substancesinclude everyday materials such as starch, naphthalene (moth balls),quinine and a host of other aromatic organic compounds and inorganiccompounds which could obscure the original imprint or code, or add afalse reading which would confuse a photosensitive readout.

STATEMENT OF THE INVENTION In accordance with the present invention, ithas now been found that a document can be identified by the embodimenttherein ofa thermoluminescent phosphor, preferably in the form of acoded message, such as a number of several or more digits, and theauthenticity or identification of the document can be verified byactivating said phosphor by exposure to appropriate radiation, andthereafter determining whether radiation is discharged from saidactivated document, generally at a temperature above that at which thedocument is exposed to the activating radiation. Such identifying meansor method can be used on various valuable bonds, currency, credit cards,identification cards and the like. Equipment for activating andidentifying such documents can be installed in banks, department stores,hotels, gasoline stations, etc., and can be of simple and less expensivetypes as well as more elaborate and possibly more expensive types.

Thermoluminescent (T.L.) phosphors are a special class of luminescentmaterials first noticed by Farrington Daniels at the University ofWisconsin. This discovery is disclosed in U.S. Pat. No. 2,616,051, datedOct. 28, 1952. Their emissions have frequencies characteristic to theindividual T.L. phosphor. These materials differ from other phosphors inthat they are capable of storing a significant percentage of theexciting energy for long periods of time until they are heated tocritical temperatures which are specific to each material, at whichtemperatures the stored energy is released or discharged spontaneouslyin the form of radiation (usually at a longer wave length than theoriginal exciting energy). These critical temperatures generally occurat higher than the ambient temperatures usually encountered, and in anycase higher than the temperature needed to excite them to a state ofpotential fluorescence. However, it has been determined that some T.L.phosphors have readout, or emission, temperatures at less than usualroom temperature ambients. One example is zinc oxide activated with zinc(ZnO which has emission peaks below room temperature as well as above;and has an afterglow (after the stimulating radiation has been removed)corresponding to a half life of seconds.

T.L. phosphors are occasionally encountered in natural minerals (forinstance, calcium sulfate and calcium fluoride) with suitable amounts ofaccidental impurities which activate the otherwise inactive latice sothat the phenomenon of thermoluminescence is present. More efficientT.L. phosphors have been prepared synthetically by growing crystals ofpurified materials into which controlled amounts of activators ordopants have been introduced into the crystal latice. Two of the moreconvenient of these, for the purpose of the application disclosedherein, are synthetically made lithium fluoride (LiF), doped withvarious materials, and calcium sulfate doped with Manganese (CaSO Theformer material is described in U.S. Pat. No. 3,320,180, issued May 16,1967 to Carl F. Swinehart. The activators, or dopants, are manganese(Mn), Calcium (Ca), barium (Ba), aluminum (Al), titanium (Ti) oreuropium (Eu). The main product of commerce is denoted as T.L.D.l00,available from the Harshaw Chemical Co., Division of Kewanee Oil Co.,and is one of the materials suitable for use in the practice of thepresent invention.

A second material, (CaSo., suitable for use in the practice of thisinvention is prepared according to the method described in LuminescentDosimetry, A.E.C. Symposium, Series No. 8; Clearing House for FederalScientific and Technical Information; National Bureau of Standards; U.S.Dept. of Commerce, Springfield, Va., Doc. No. CONF: 650637; Pg. 205,Thermoluminescent Readout Instruments for Measurement of Small Doses, by.I. Lippert and V. Mejdahl. Material prepared by the procedure describedtherein is suitable for use in the practice of this invention.

Both of the above materials are presently available commercially andtheir cost is within a range to make them economically feasible for theuses disclosed herein. Other materials in the class of T.L. phosphorsmay be available in the future.

Up to this time these T.L. materials have been used primarily in thefields of radio-medicine and physics. In medicine they provide aconvenient means of determining and preserving a quantitative record ofthe amount of radiation from gamma and x-ray sources to which a patienthas been subjected, either deliberately as a therapeutic treatment, oraccidentally as an occupational hazard. In the field of physics they areused for purposes similar to the above, but also for other purposes suchas mapping radiation patterns around nuclear reactors. The sole purposeof these prior uses is to determine the quantitative exposure toradiation of an object, either in a single, or multiple or continuousexposure.

In the present invention the main objective is to determine if such aT.L. material is present in or on a document, ID. card or other objectas a means of determining its authenticity or officiality by means of acoding system. The specific quantitative exposure is relativelyimmaterial, so long as it is strong enough to satisfy the parameters ofthe code. The simplest code is the presence or non-presence in aquantity significantly above background or casual, accidentalcontamination.

In the applications herein claimed the exciting radiation need not begamma or x-rays, but for some materials can be a simple ultra-violetsource.

Another significant difference from previous applications of T.L.materials is that for the simple confirmation of their presence the usercan activate the T.L. materials immediately prior to the readout (orconfirmation procedure), thereby eliminating any significant energy losswith time, or energy storage from accidental exposure to excitingradiation. T.L. materials lose various amounts of stored energy withtime, a typical case being T.L.D.l which dissipates about 5 percent peryear. Their stored energy can also be augmented by unintentionalexposure to x-ray or gamma ray sources.

For most purposes of this application, long-term storage properties, interms of pickup or loss characteristics, are of no consequence. In otherwords, previous uses have been confined to a means of detecting levelsof radiation to which an object has been exposed, whereas thisdisclosure deals with a means of confirming the presence of T.L.phosphors according to some code or configuration which is appropriateto the degree of complexity desired, and gains or losses with time areeliminated. The coding choices are discussed below.

The exposing radiation levels are of no significance to the user so longas they are strong enough to excite the phosphor to a point where theeventual emission, upon heating, confirms its presence by a sufficientmargin to discriminate against casual or accidental contamination.

T.L. phosphors can be applied to an object in a variety of ways. In thecase of paper, the phosphors in fine powder form can be incorporatedinto the paper itself so long as enough is present at or near thesurface so that the emitted light is not absorbed below detectablelevels. T.L. powders can also be printed onto the surface of any objectwith a binder that is essentially transparent to the wave length ofemitted radiation. In the case of plastics or other objects which areusually formed by casting or molding, in addition to the above methods,small chips or aggregates of T.L. material can be incorporated into thesurface or in the body, if the body is sufficiently transparent to theemitted radiation.

It is not necessary to deal with the specific T.L. phosphors which aremost suitable for each specific type of material to be authenticated, orthe specific design of the detector or readout" equipment to beemployed. However, a general discussion of these is in order.

At the present time readout equipment for T.L. phosphors, or readers,consist of a light-proof drawer or compartment in which the sample isplaced, an electrical resistance heater placed so that the sample iswarmed to the desired temperature, a light-sensitive device to detectthe emitted radiation, and a means of displaying this signal. At presentthe detector employed is a photomultiplier tube (RM. tube) which iscapable of multiplying the incident energy many thousands of times. Sucha detector can register energy far below visual levels. Other types ofdetectors are available, such as silicon photo-transistors and othersolid state energy converters. These are presently less sensitive thanP.M. tubes, but could be practical. For some applications, high speedphotographic film could be used as the detector. This is morecumbersome, but has the advantage of being able to record specialconfigurations, or patterns quite simply.

Obviously, the T.L. phosphor selected must be one such that it possessesa low enough emission or readout temperature so that the article to beauthenticated will not be damaged by the heat necessary to release thestored energy in the form of detectable radiation. T.L. materialsusually have several emission levels in terms of temperature. Forinstance, T.L.D.-l00 has several in a range from C. to 245 C. Since oneof the strongest emissions occurs at 80 C., this is a convenientmaterial with which to authenticate or tag paper or plastic documentsbecause neither the paper, plastic, nor ink will be damaged at thisemission temperature if the proper materials are chosen. With as littleas l milligram in the field of view of a photomultiplier tube, a readingof approximately 10 times the intensity of untreated paper is obtainedwith an exciting exposure of l roentgen. Higher exposure levels canreduce the quantity of T.L. phosphor needed, per unit of area.

The phosphor CaSO has an even higher emission for the same exposure doseand can therefore be used in smaller concentrations per unit of area tobe surveyed by a detector in order to obtain a significantdifferentiation from ambient and non-intentional signals. It also has alow emission peak.

In some cases the simple verification of the significant presence of aT.L. phosphor, as opposed to their casual presence on a document, I.D.card, or other object, may not be as sophisticated or encoded as theuser might wish, from the standpoint of guarding against unlikely butpossible coincidences. In such cases it is relatively simple to useseveral different T.L. phosphors which have emissions at different wavelengths, and to apply these in a coded pattern such that the probabilityof an accidental coincidence is reduced by several orders of magnitude,and to a point where it would be nearly statistically impossible toduplicate without the pattern code combined with the wave lengthemission code in the proper sequence or position.

Since P.M. tubes, and most other detectors, cannot discriminate one wavelength from others within the wave length range or frequency band towhich they are sensitive, it is necessary in the above example toprovide some means of decoding a pattern caused by the use of severalT.L. phosphors in different spatial configurations, each of which isemitting its own frequency (or wave length). This is convenientlyaccomplished by using band pass filters with patterns or windowscoinciding with the original coded design, such that the filter willpass the proper frequency either in (1) the proper sequence, or (2) theproper spatial configuration, or (3) the proper integrated total of allsignals. If the readout is done by method (1) above, a single detectorcan be used with a suitable filter to read out the first phosphor; thenthe filter is changed to read out the second phosphor, etc. This wouldbe the case where all phosphors are mixed together or located injuxtaposition such that the field of view of the RM. tube does not needto shadow mask in register with the location of the various phosphors.

In example (2) above, several different filters may be located in ashadow mask such that each filter is in register with the spatiallocation of its corresponding T.L. phosphor. In this case, either asingle detector can be moved from one location of phosphor to the next,taking a separate reading each time, or a multitude of detectors can beused, each viewing only one phosphor location, thereby reading alllocations simultaneously but also keeping the readings separate. Thiswould be quicker but more expensive in terms of equipment.

Example (3), above, would use the same shadow mask described in example(2), but a single P.M. tube would view all locations simultaneously anda single readout would not keep these signals separate but integratethem into a single reading to coincide with the coded input.

As a simpler alternate to examples (2) and (3) above, the same phosphorcan be used in different concentrations in each location, eliminatingthe need for differentiating band pass filters. In other words, thespatial coding can be accomplished by the concentration and, therefore,the intensity of emission, at various spatial locations, but all at thesame wave length. Therefore, one has the choice of coding by presence ornonpresence, augmented by amplitude modulation and frequency modulationand spatial modulation, and any combination of the above. The choiceamong these more complicated coding systems is simply a question ofbalancing the cost with the need for certainty that the code has notbeen broken by comprehension, or confused accidentally.

In experiments with US. paper currency, sections of Federal Reservenotes of approximately 2 square centimeters are used to establish casualand incidental background radiation levels. The readout temperature is85 C. and the readout time is 30 seconds. This readout time isarbitrarily selected in order to make sure that all the available energywill be collected, but in actual practice the readout time can bereduced substantially since most of the signal is released within 2seconds. The readings are as follows, as obtained on a Harshaw ChemicalT.L.D. Reader, Model 2000:

1. Using no sample at all the background radiation or dark currentenergy is 0.043 nano-coulombs from two different specimens.

2. A section of a used One Dollar ($1.00) bill gives a reading of 0.050nanocoulombs.

3. A section of a new Twenty Dollar ($20.00) bill gives a reading of0.049 nano-coulombs.

4. Repeat readings of example (3) give 0.046 nanocoulombs on the secondreading, and 0.046 on the third reading.

5. Example (4) is repeated with an addition of a fresh thumb print 'andthe reading is 0.049 nanocoulombs. In addition, floor dust, by wipingthe sample on the floor, gives a reading of 0.046 nanocoulombs.

6. The addition to example (5) above of I milligram of T.L.D.l00 as thephosphor, gives a reading of 0.47 nano-coulombs, or over times thebackground.

results are obtained:

Charge or Dark Current 0.015 nano-coulombs Peak Current [.SXIO" ampsItem no sample (to establish equipment background) 1 -NMIU) (activated)1.9 nanocoulombs l000 l0 amps In the above case the total charge isapproximately 129 times the dark current energy, and the peak current isapproximately 667 times the peak dark current. For this material thepeak current is therefore a more sensitive measurement than totalcharge.

For purposes of this disclosure, the use of T.L. materials as a means ofauthenticating articles or objects as to a predetermined code, it isdifficult to set specific quantitative parameters as to minimum amountsof T.L. phosphors to be applied to the article or object. The factorsgoverning such a minimum quantity will depend on many factors, includingthe following:

1. The specific T.L. phosphor employed.

2. The background thermoluminescence (if any) of the object to which itis applied.

3. The specific input, or activating energy.

4. The efficiency of the detector system used to survey the dischargeemissions at the temperature which is practical for the object ormaterial to be verified or authenticated.

5 The output of secondary emission energy, or, alternatively, the peakcurrent, on discharge as a function of the primary or activating energy.(This is the efficiency factor of the specific T.L. phosphor.)

6. The readout time, at or above the discharge emission temperature.

All of the above parameters add up to defining a total system. In orderto obtain a significant readout signal for any total system, asdescribed above, the output of secondary emission in terms of energy orpeak current measurement for a given system when the T.L. phosphor isactivated, should exceed the same measurement for the same system,inactivated, by a factor of at least two 2. It would be preferable tohave this factor higher, and in actual practice it can easily beexceeded by many times, but a factor of two should generally be enoughto discriminate against accidental surface contamination.

The maximum factor, as defined above, is not material, except that it isdesirable, in the interest of security, that the readout energy, orintensity, be less than that which could be visible to the naked eye ina dark room, so that the presence of T.L. phosphors is not obvious.

In some cases it may be useful to take advantage of the long termstorage capabilities of some phosphors or the relatively short termstorage capabilities of others. As mentioned before, T.L.D. loses about5 percent of its charge per year. However, (CaSO has a half-life (loseshalf its charge) of one hundred 100 hours.

Some T.L. phosphors are capable of giving off emissions of a part of theactivating energy shortly after activation even without heating. Suchphosphors can therefore be read at ambient temperatures for asubstantial period after activation.

In the case of long term storage, such materials can be used to detectstolen airline tickets or other documents which are used only once.Contrary to the previously mentioned applications, where the taggedobject is activated immediately prior to readout, a ticket can beactivated, and thereby validated when delivered to a customer. It wouldtherefore contain a long life message which can be read out at the timeof presentation to the carrier. Unissued tickets would have no message.Other examples of such use are numerous.

In the case of long or short term storage, such materials can be usefulin determining the age of a tagged material. For instance, manymaterials lose their usefulness, or become dangerous after a certainlength of time. Food products and medicines are two prime examples.Tagging labels with a T.L. material of suitable half-life can provide apositive means of establishing age.

For example, in order to verify the authenticity of a credit card, anarbitrary or random number is assigned to the owner of the card which isknown to him alone (other than the issuer of the card) and does notappear on the card except as a coded pattern as described below. Thisnumber is composed of as many digits as can conveniently be remembered,such as five or six digits. In terms of binary digital language, theformula for onoff". yes-no or go-no-go signals commonly called bits, isa function of two to the nth power (2"). In specific terms 2 equals16,384, 2 equals 131,072, etc. Therefore, with 14 on-off" combinationsone can count to 16,384 in binary language. This should be enough todiscriminate against the coincidental guessing of a number not known tothe user of a credit card.

For this purpose such a number is easily encoded into the card byembedding very small quantities of a suitable T.L. material into orunder the surface of the card, so that it is not easily rubbed off. Thequantities required for each bit are very small and should beunnoticeable, since most T.L. materials are essentially transparent. Itis practical to arrange at least 18 or more such signal generators inone continuous line, horizontally or vertically or otherwise on theusual credit card dimensions without having them spaced too closely forreadout purposes. The on signal would be a small dot of T.L. phosphor.The of signal would be either a blank space or a dot of the samechemical compound without the dopant which causes it to emitsignificantly detectable radiation when it is above the readouttemperature.

In the above example the specific wave length can be ignored as long asthe detector system will respond to it, and the specific amplitude isnot important as long as it is enough to discriminate against accidentalcontamination as discussed above.

The readout mechanism can consist of a single P.M. tube, or othersuitable detector, which is arranged so as to have a sufficiently narrowfield of view so that it can see" only one dot or signal location at atime. The

credit card can be inserted into a box which contains:

1. A source of energy to activate the T.L. phosphor,

i.e., ultra-violet, x-ray, or gamma-ray source.

2. A source of heat, if necessary, for readout of the T.L. phosphoremployed. (See (6) below.)

3. A means for the card-owner, or customer to dial or punch-in (via discor keyboard) his own secret identification number.

4. A means of storing the above signal for comparison with the signalcreated by reading the card with a detector as described below. Suchcomparison circuits are well known in electronics.

5. A P.M. tube or other detector which can see or read only one signalat a time from the card.

6. A mechanical means of advancing or moving the card from one signallocation to the next. This can be a spring loaded device which isactivated by insetting the card and ejects the card in a ratchet typemovement, so that each signal or bit is positioned under the detector insequence. In this example it is simple to select a T.L. material thatreads out below ambient temperatures and has a persistent afterglow, orhalf life, long enough for readout of all signals without needing aheater.

7. A readout signal which could consist of one red and one green lighttube or window mounted so as to be visible to the clerk or salesperson.One of these lights would indicate that the numbers coded into thedevice by the customer matches the numbers coded into the device by thesignals on the card. The other light would indicate that they do notmatch. (This process is handled by the comparison circuit mentioned in(4) above.)

This example is only one of many possibilities for variations on theprinciples included in the claims for systems which follow.

A method of practicing this invention is illustrated by the followingexample. This example is intended merely to illustrate the invention andnot in any sense to limit the manner in which the invention can bepracticed. The parts and percentages recited therein and all through thespecification, unless specifically provided otherwise, are by weight.

EXAMPLE A laminated plastic credit card is assembled and sealed with anumber of bits or traces of T.L.D.-1OO phosphor embedded in anappropriate pattern to give the number 167,489 in accordance with thearrangement shown below where the Xs indicate the location of a trace ofphosphor; and the Os indicate a blank or false phosphor. (To code theabove number requires 18 bits when one dimensional coding is employed):

This series can represent any number up to 2 but can be assigned thevalue 167,489 in a master code. The cured or sealed card is placed in anactivating radiation field and exposed to l roentgen of gamma radiationin a few seconds and then in a readout device having a single P.M. tubeas described above which surveys each location in sequence. The presenceor absence of light emission above a pre-set level is distinguished bythe detector. Impulses from the detector are matched to the code storedin the devices logic circuit and the corresponding number is recognized.This number can be displayed visually and compared to the number givenby the user; or preferably not displayed, but compared electronically toa number dialed or keyboarded into the reader by the cardholder so thatthe actual number is not made visible to anyone.

While certain features of this invention have been described in detailwith respect to various embodiments thereof, it will, of course, beapparent that other modifications can be made within the spirit andscope of this invention and it is not intended to limit the invention tothe exact details shown above except insofar as they are defined in thefollowing claims:

The invention claimed is:

1. A process for identifying a document comprising the steps of:

a. embedding a small amount ofa thermoluminescent phosphor in one ormore small confined areas of the document, said areas being arranged togive a coded message upon subsequent readout;

b. exposing said document to an activating radiation for said phosphorat a temperature at which the energy imparted by activating radiationwill not be immediately discharged;

c. thereafter positioning said document in a readout device capable ofinterpreting the coded message; and

d. increasing the temperature of said document to a temperature at whichsaid phosphor will discharge the radiation energy stored therein.

2. The process of claim 1 in which said activation is effected at roomtemperature.

3. The process of claim 2 in which the temperature in said readoutdevice is increased to about -85 C.

4. The process of claim 1 in which the amount of phosphor in differentareas is varied to give different amplitudes of energy upon readout.

5. The process of claim 1 in which said message is encoded by using twoor more thermoluminescent phosphors with different emission frequencies.

6. The process of claim 5 in which said message is coded and decoded bythe spatial arrangement of said different phosphors.

7. The process of claim 5 in which said message is coded and decoded bythe use of band-pass filters to separate the emission from saidphosphors.

8. The process of claim 1 in which said document is dated by the use ofa phosphor having a half-life appropriate for the time lapse desired tobe identified.

9. The process of claim 1 in which said phosphor is one that dischargesat least a portion of the activating energy at ambient temperature sothat deliberate heating of the document is not required.

10. An identifiable document having a small amount of thermoluminescentphosphor embedded in one or more small confined areas of said document,the number and positioning of said areas being designed by predeterminedcode to give specific identifying information, said phosphor beingcapable of activation by radiation at room temperature and capable ofdischarging the stored radiation energy therein and thereby theidentifying information upon increasing the temperature of saiddocument.

1. A process for identifying a document comprising the steps of: a.embedding a small amount of a thermoluminescent phosphor in one or moresmall confined areas of the document, said areas being arranged to givea coded message upon subsequent readout; b. exposing said document to anactivating radiation for said phosphor at a temperature at which theenergy imparted by activating radiation will not be immediatelydischarged; c. thereafter positioning said document in a readout devicecapable of interpreting the coded message; and d. increasing thetemperature of said document to a temperature at which said phosphorwill discharge the radiation energy stored therein.
 2. The process ofclaim 1 in which said activation is effected at room temperature.
 3. Theprocess of claim 2 in which the temperature in said readout device isincreased to about 80*-85* C.
 4. The process of claim 1 in which theamount of phosphor in different areas is varied to give differentamplitudes of energy upon readout.
 5. The process of claim 1 in whichsaid message is encoded by using two or more thermoluminescent phosphorswith different emission frequencies.
 6. The process of claim 5 in whichsaid message is coded and decoded by the spatial arrangement of saiddifferent phosphors.
 7. The process of claim 5 in which said message iscoded and decoded by the use of band-pass filters to separate theemission from said phosphors.
 8. The process of claim 1 in which saiddocument is dated by the use of a phosphor having a half-lifeappropriate for the time lapse desired to be identified.
 9. The processof claim 1 in which said phosphor is one that discharges at least aportion of the activating energy at ambient temperature so thatdeliberate heating of the document is not required.
 10. An identifiabledocument having a small amount of thermoluminescent phosphor embedded inone or more small confined areas of said document, the number andpositioning of said areas being designed by predetermined code to givespecific identifying information, said phosphor being capable ofactivation by radiation at room temperature and capable of dischargingthe stored radiation energy therein and thereby the identifyinginformation upon increasing the temperature of said document.